Fabric softeners provide a means to impart a variety of desirable characteristics to clothing, the most obvious being improved feel when the fabric is rubbed across the skin. Through the use of perfume or masking scents, fabric softeners can also impart a perception of freshness. In addition, fabric softeners provide a delivery vehicle for attaching other consumer-beneficial additives, such as soil release agents, whitening agents, antiwrinkling agents, dye transfer inhibition agents, color protection agents, and fabric care agents.
The history of fabric softeners in consumer use is associated with the conversion of laundry detergents from tallow-based soaps to synthetic bases. Since ancient times, clothes have been washed with soaps (sodium salts of fatty acids) by hand, and later with a mechanical washing machine. Around 1945, synthetic detergents, primarily based on alkylbenzenesulfonates as well as other secondary surfactants began to rise in prominence for machine washing in North America. The new generation of laundry detergents was formulated with builders, that is, sequestering agents such as phosphate, carbonate or citrate, to reduce the deposition of insoluble calcium and magnesium salts of soap and alkylbenzenesulfonates. These insoluble calcium and magnesium salts cause redeposition of soil, resulting in a gradual buildup of a dingy, gray film on light-colored fabrics.
The presence of sequestering agents resulted in a significant reduction in the amount of lime soaps left behind on clothes. Moreover, mechanical washing machines coupled with improved detergent formulations led to improved removal of oils, clay soils, and other natural fiber lubricants. These residues all contributed to a softer hand and their enhanced removal resulted in a harsher feel of the fabric.
Cotton, still the predominant fiber in today's textile industry, suffers from unique mechanical wear and tear processes which ultimately create consumer demand for fabric softeners. With repeated laundering, cotton microfibrils break and unravel. Mechanical friction in the washing process induces static charges that cause the microfibrils to project orthogonally from the fiber bundle upon drying. These microfibrils act as barbs which inhibit fiber-fiber slippage, interfere with fiber flexibility, and are perceived as a sources of a drag when drawn across the skin. All of these phenomena contribute to the total perception of roughness. Softening materials can reduce fiber-fiber interactions by reducing static and allowing microfibrils to lay parallel to the fiber bundle and/or by coating and lubricating the fiber bundle to minimize friction. Further, they can provide a lubricating layer between the fiber surface and human skin. The net result is the perception of a less abrasive, more pliable fabric.
Cationic surfactants are the most common ingredients used worldwide as rinse-added fabric softeners. The reasons for this are many. They are cost-effective, being highly efficient at depositing or "exhausting" onto the fabric even at extremely low concentrations. They are effective at reducing microfibril static and interfiber friction. They provide a renewable finish that interferes only minimally with the laundering process. They are is based on low-cost raw materials, predominantly tallow, lard, or alternatively, on seed oils such as palm oil, soybean, or canola (rapeseed) oil. They are relatively easy to formulate with conventional mixing equipment and require few supplemental ingredients. They are essentially nontoxic to higher life forms. They are ultimately biodegradable and do not build up in the environment.
It is well known that controlling the rheology and physical stability of cationic softener formulations is difficult. This is due to the fact that cationic surfactants are disrupted and rendered ineffective by a wide range of materials. Anionic species, either dissolved or suspended may adsorb or precipitate the surfactant, causing both rheological and physical instability i.e. the product may become too thick or too thin, or phase separation of the aqueous phase may occur. Thus, unless used to form neutral fatty softening species or to deliberately thin the formulation e.g. liquid concentrates, anionic surfactants and additives are avoided by the industry. The formulations cannot therefore be thickened using anionic polymer thickeners.
Many current fabric softener compositions use heteropolysaccharides such as xanthan gums as rheology modifiers. The xanthan gums are dry materials and therefore require a make down step to slurry or disperse the material into the fabric softener composition. In addition, xanthan gums are a source for microbial growth. Microbial contamination causes a loss of viscosity in the fabric softener composition and subsequent spoilage of the product.
U.S. Pat. No. 5,114,600 describes a fabric conditioning formulation containing a cationic softener and a cross-linked cationic polymer which is prepared from an ethylenically unsaturated monomer which is crosslinked with 5 to 45 ppm of a cross-linking agent. U.S. Pat. No. 5,869,442 describes a fabric softening composition containing a polyvinylpyridine betaine containing a quaternary nitrogen and a carboxylate salt. PCT application WO 99/06455 describes crosslinked cationic homopolymers as thickening agents for acidic laundry softeners. The crosslinking agent is present in an amount of from not less than 50 to 600 ppm of the homopolymer total weight.
There continues to be a need for controlling the rheology and physical stability of cationic softener formulations without a make down or slurry step prior to dispersing the rheology modifier in the fabric softener.